Synchronous rectifier that is impervious to reverse feed

ABSTRACT

A synchronous rectifier having two MOSFETs and a transformer with a primary winding and a first and second secondary winding in the transformer and a control device in the rectifier circuitry. The second secondary winding (tertiary winding) and the control device are connected within the synchronous rectifier to provide protection for the two MOSFETs from destruction caused by reverse feed.

BACKGROUND

The present invention relates to a synchronous rectifier, that isimpervious to reverse feed.

Feeder devices with synchronous rectifiers are known per se, and are inuse as electric power supply devices. In the same way, such devices areknown from publications, for example, from O. Kilgenstein,Schaltnetzteile in der Praxis, Wuerzburg, 1992. Each of the knowndevices has a range of operation that is limited at the top by themaximum output current at which the output voltage is still regulated tothe desired value.

If either temporarily or for long periods the current requirement isgreater than the capacity of a single device, and the selection of asimilar device of greater capacity is not indicated for certain reasons,then these devices are connected in parallel.

If in one of the devices that is connected together in parallel in thismanner, an operating state arises in which either the output voltagedrops or the oscillator no longer triggers or interrupts the primarytransformer, then the devices that are connected in parallel with thedevice under consideration feed power back into the device that iseither temporarily or permanently out of service, which as a rule leadsto the destruction of the controlled rectifier or to a collapse of theoutput voltage.

SUMMARY

The task of the present invention is to create a synchronous rectifierthat is impervious to reverse feed, and that is fully capable ofoperation in all operating states. In one embodiment, the presentinvention comprises a synchronous rectifier, including a transformer (3)with a primary winding (2), which is supplied on an alternating basiswith current from a direct-current source (1) by means of a switch (4)that is controlled by an oscillator, which transformer (3) exhibits afirst secondary winding (5) with ends (C, D), additionally including afirst MOSFET (6) and a second MOSFET (7), which are connected into thecircuit on an alternating basis in time with the oscillator, whereby theone MOSFET (7) fulfills the function of a rectifier diode, and thesecond MOSFET (6) that of a recovery diode, the drain electrode of thefirst MOSFET (6) is joined on the one hand with the one end (C) of thefirst secondary winding (5), and on the other hand, with an output (14)of the apparatus by means of a storage choke (12), the drain electrodeof the second MOSFET (7) supplies current to the other end (D) of thefirst secondary winding (5), the source electrodes of both MOSFETs (6,7) are joined with a line (11), and by means of this line, with theother output (15) of the apparatus, and the two outputs (14, 15) of theapparatus are joined by means of a storage and smoothing capacitor (13),characterized by the fact that: the transformer (3) exhibits a secondsecondary winding (16) with ends (E, F), whereby the one end (E)exhibits the same polarity the as one end (C) of the first secondarywinding (5); a control device (17) is present, which exhibits sevenconnections (21 through 27), whereby the first connection (21) isconnected with the one end (E) of the second secondary winding (16), thesecond connection (22) with the second end (F), the third connection(23) is connected with the one end (C) of the first secondary winding(5), the fourth connection (24) is connected with the other end (D) ofthe first secondary winding (5), the fifth connection (25) is connectedto the line (11), the sixth connection (26) to the gate electrode of thefirst MOSFET (6), and the seventh connection (27) to the gate electrodeof the second MOSFET (7); and the control device (17) contains means forthe determination of whether a voltage is being induced through thefirst and second secondary windings (5, 16), and based on that, whetherthe synchronous rectifier is working at all, and in the event of anegative case, additionally contains a means for blocking the twoMOSFETs (6, 7), whereupon these MOSFETs can continue to carry out theirdiode function by means of their substrate diodes.

In another embodiment of the present invention, a synchronous rectifiercomprises a direct-current current source (1), whose flow of currentthrough the primary winding (2) of a transformer (3) is periodicallyswitched on and off by means of an electronic switch (4)--by anoscillator that is not shown--contains as a controlled rectifier twoMOSFETs (6, 7). If two or more such synchronous rectifiers are connectedtogether in parallel, it is necessary and in accordance with theinvention that, in the event of the failure of such a synchronousrectifier, the connected MOSFETs (6, 7) are protected from destructionresulting from reverse feed from this rectifier or the remaining ones,This is achieved by means of the insertion of a control device (17) withseven connections (21 through 27) and a second secondary winding (16) ofthe transformer (3); while a first secondary winding (5) of thetransformer (3) takes over the actual power supply of the synchronousrectifier, and supplies current to the connection (23) with one end (C)and to the connection (24) with the other end (D), each of which ends(C, D) also supplies current to the drain of one of the MOSFETs (6, 7),the second secondary winding (16) with ends (E, F) has what is primarilya control function, and supplies current to the connections (21, 22)from one end each (E, F). The gates of the MOSFETs (6, 7) are connectedone each to the connections (26, 27), and are connected within thecontrol device (17) with the sources of one additional MOSFET (36, 37)each. The gates of the MOSFETs (36, 37) are triggered from connection(21) by means of the series wiring of a resistor (30) with a diode (31),and are connected on the other side by means of the parallel wiring of aresistor (32) with a capacitor (33) to connection (25), which a line(11) connects with an output (15) of the synchronous rectifier.Connected to the line (11) as well are the sources of the MOSFETs (6,7), whose gates are also connected by means of resistors (35, 34) withthe connection (25). Between the drain of MOSFET 6 and the other output(14) of the synchronous rectifier there is a storage choke (12); the twooutputs (14, 15) connected by means of a storage capacitor (13).

BRIEF DESCRIPTION OF THE DRAWINGS

With the aid of the attached drawing, the invention is explained in moredetail by means of several embodiments. The following are shown:

FIG. 1 shows a schematic diagram illustrating a part art rectifiercircuit;

FIG. 2 shows a schematic diagram illustrating one embodiment of thepresent invention;

FIG. 3 shows a schematic diagram of the device in FIG. 2, illustratingone embodiment of the control device therein; and

FIG. 4 shows a schematic diagram of the device in FIG. 2, illustratinganother embodiment of the control device therein.

DETAILED DESCRIPTION

In FIG. 1, the state of the art that forms the starting point is shown.A direct-current current source 1 feeds the primary winding, designatedby 2, of a transformer 3. The current that supplies the primary winding2 is periodically interrupted by an electronic switch 4, as a result ofwhich an--approximately rectangular current and voltage pulse arises inthe secondary winding 5 of the transformer 3. Two MOSFETs 6, 7, thegates of which are triggered by two resistors 8, 9, are used ascontrolled rectifiers, and supply two lines 10, 11 with current in phasewith the switch 4. The line 11 is connected directly toa--negative--output 15, while between an additional--positive--output 14and the drain electrode of MOSFET 6, a storage choke 12 is connected.The resistor 8 is connected to the secondary winding 5 at end C, theresistor 9 at end D. The end C of the secondary winding 5, theconnection of the resistor 8, the drain electrode of MOSFET 6, and oneof the connections of the storage choke 12 form one node, while the endD of the secondary winding 5, the connection of the resistor 8, and thedrain electrode of MOSFET 7 form an additional node. The sourceelectrodes of the two MOSFETs 6, 7 are connected to the line 11. Betweenthe outputs 14, 15 there is a capacitor 13 that has a smoothing andstorage function.

This circuit requires that a passive load be applied between the outputs14, 15. if, however, two or more rectifiers of the type that has justbeen described are connected together in parallel, it is thenpossible--for example, if the switching function of switch 4 were tofail briefly--for current to flow back into the circuit, which wouldlead to a short circuit through the MOSFETs 6, 7, which are nowconductive, and possibly to their destruction. The two so-calledsubstrate diodes of the MOSFETs 6, 7 are shown as well by means ofdashed lines.

The basic circuit diagram of the solution in accordance with theinvention is shown in FIG. 2. The transformer 3 exhibits a thirdwinding, the second secondary winding 16, whose polarity relative to thefirst secondary winding 5 is such that the end C of the first secondarywinding 5 exhibits the same polarity as an end E of the secondarywinding 16. The polarity of an end F of the secondary winding 16 thencorresponds to the polarity of the end D of the first secondary winding5. In addition, the circuit in accordance with FIG. 2 also includes acontrol circuit 17, which, in accordance with the invention, containsthose circuit elements and in that combination which makes up thesolution in accordance with the invention.

The circuit 17 exhibits seven connections 21 through 27 by means ofwhich it is connected both to the known circuit in accordance with FIG.1, as well as to the second secondary winding 16: the connections 21 and22 produce the connection with the second secondary winding 16, that is,its ends E, F, the connections 23, 24 produce the connection with thefirst secondary winding 5, that is, its ends C, D. The connections 26,27 control the gate electrodes of the MOSFETs 6, 7, while connection 24is linked with the drain electrode of MOSFET 7, and connection 25 withthe line 11 to which the source electrodes of both of the MOSFETs 6, 7are connected.

FIG. 3 contains the wiring diagram of a first embodiment of the circuit17 in accordance with the invention. The end E of the second secondarywinding 16 is joined by means of a resistor 30 to a diode 31, which isjoined on the other side with the gate electrodes of two additionalMOSFETs 36, 37. These gate electrodes that have just been mentioned areadditionally joined, by means of a resistor 32 that is wired in parallelwith a capacitor 33, with the line 11 by means of the connection 25. Theend F of the second secondary winding 16 is likewise--within the controlcircuit 17--attached to the line 11, which in turn is joined by means ofa resistor 34 by means of the connection 27 with the gate electrode ofthe MOSFET 7, and by means of a resistor 35 by means of the connection26 with the gate electrode of the MOSFET 6. The drain electrode of theMOSFET 36 is joined by means of the connection 24 with the end D of thefirst secondary winding 5 and the drain electrode of the MOSFET 7, andthe drain electrode of the MOSFET 37 is joined by means of theconnection 23 with the end C of the first secondary winding 5 and thedrain electrode of the MOSFET 6.

As long as a passive load is connected between the outputs 14, 15, thecontrol circuit in accordance with FIG. 3 works, in terms of its effect,like the known circuit in accordance with FIG. 1:

The gate electrodes of the MOSFETS 6, 7 can be charged only if thecapacitor 33 exhibits a positive voltage with respect to line 11, sothat the MOSFETs 36, 37 are conductive. This is brought about by theoutput voltage U_(EF) of the second secondary winding 16, which appearsonly if the transformer 3 is being supplied with pulses by means ofswitch 4. A charging current for the gate electrodes of the MOSFETs 36,37 and the capacitor 33 flows through the resistor 30 and is limited byit. During the change of polarity of the voltage U_(EF) s, the diode 31prevents the discharge of the gate capacitances and the capacitorthrough the second secondary winding 16. In addition, the capacitor 33maintains the voltage of the gate electrodes of the MOSFETs 36, 37 atthe necessary level if U_(DS) (the voltage between drain and source) ofthe MOSFETs 36, 37 changes.

If, on the other hand, the switch 4 malfunctions and U_(EF) collapses,the capacitor 33 will discharge through the resistor 32, the MOSFETs 36,37 block, and the gate electrodes of the MOSFETs 6, 7 will dischargethrough the resistors 34, 35: the MOSFETs 6, 7 likewise block. Essentialto the invention is now the fact that the substrate diodes of theMOSFETs 6, 7 maintain the normal rectification function, although,because of their large forward voltage, with a decreased overallefficiency of the apparatus.

In the second embodiment, in accordance with FIG. 4, the two MOSFETs 36,37, which control the gate electrodes of the MOSFETs 6, 7, are wireddifferently with respect to their gate and drain electrodes. The twogate electrodes of the MOSFETs 6, 7 are both connected by means of thecapacitor 33 to the connection 25, which is joined with the line 11.These pooled gate electrodes are triggered, by means of a diode 41 andthe resistor 42 that is connected with it in series, from the signalthat is generated at the end C of the first secondary winding 5. This isfed into the control circuit 17 by means of the connection 23. The drainelectrode of the MOSFET 36 is joined by means of a zener diode 43 withthe connection 24, and this is joined with the end D of the firstsecondary winding 5. The end F of the second secondary winding 16 isjoined directly to the line 11 by means of the connections 22 and 25,while the end E of the second secondary winding 16 supplies--by means ofthe connection 21--current to the drain electrodes of the MOSFET 37.

If the switch 4 opens and closes at the desired timing frequency, thenthe effect of the control circuit 17 in accordance with the invention isagain the same as that of the known circuit in accordance with FIG. 1:When the switch 4 closes, voltages U_(CD) and U_(EF), which are in thesame direction, occur by means of the secondary windings 5 and 16. Thecapacitor 33 and the gate electrodes of the MOSFETs 36, 37 are chargedby means of the diode 41 and the resistor 42--which limits the chargingcurrent; the MOSFETs 36, 37 become conductive at the appropriatedrain-sources voltage. At the change of polarity of U_(CD), the diode 41prevents the discharge of the capacitor through the first secondarywinding 6. In addition, the capacitor 33 supports the charging of thegate electrodes of the MOSFETs 36, 37 in the event of changingdrain-source voltage.

If the switch 4 now fails--while it remains open, with an active loadbetween the outputs 14, 15, the voltage that is present from outsidewill indeed be fed back in; however, the zener diode 43 reduces thisvoltage to such an extent that it is below the threshold voltage betweengate and source of the MOSFET 6, and the MOSFET 6 therefore blocks. Inaddition, the secondary winding 16 also no longer has a voltage, andthus the MOSFET 7 no longer does either. Both MOSFETs 6, 7 block, andonly the substrate diodes of the MOSFETs 6, 7 are still with an effect,as described by means of FIG. 3.

Both with the state of the art in accordance with FIG. 1 as well as theembodiments in accordance with FIGS. 3, 4 of the solution in accordancewith the invention in FIG. 2, all parts of the apparatus that bringabout the actual regulation of the voltage that is present between theoutputs 14, 15 are left out, since on the one hand, in accordance withthe state of the art such parts are assumed to be known, while on theother, they are also not affected by the control device 17 in accordancewith the invention.

What is claimed is:
 1. Synchronous rectifier, comprising a transformerwith a primary winding which is supplied on an alternating basis withcurrent from a direct-current source by means of a switch that iscontrolled by an oscillator, the transformer having a first secondarywinding with first and second ends, further comprising a first MOSFETand a second MOSFET which are connected into the circuit on analternating basis in time with the oscillator, the first MOSFETfunctioning as a rectifier diode and the second MOSFET functioning as arecovery diode, a drain electrode of the first MOSFET being joined withthe first end of the first secondary winding and with a first output ofthe synchronous rectifier by means of a storage choke, a drain electrodeof the second MOSFET supplying current to the second end of the firstsecondary winding, the source electrodes of first and second MOSFETsbeing joined by a line with a second output of the synchronousrectifier, and the first and second outputs being joined by means of astorage and smoothing capacitor, the synchronous rectifier furthercomprising:the transformer having a second secondary winding with firstand second ends, the first end of the second secondary winding havingthe same polarity the as first end of the first secondary winding; acontrol device a first connection of the control device being connectedwith the first end of the second secondary winding, a second connectionof the control device being connected with the second end of the secondsecondary winding, a third connection of the control device beingconnected with the first end of the first secondary winding, a fourthconnection of the control device being connected with the second end ofthe first secondary winding, a fifth connection of the control devicebeing connected to the line, a sixth connection of the control devicebeing connected to a gate electrode of the first MOSFET, and a seventhconnection of the control device being connected to a gate electrode ofthe second MOSFET, the control device contains means for thedetermination of whether a voltage is being induced through the firstand second secondary windings to determine whether the synchronousrectifier is working, and means for blocking the first and secondMOSFETs upon determining that the synchronous rectifier is not working,whereupon the first and second MOSFETs will continue to carry out theirdiode function by means of first and second substrate diodes.
 2. Thesynchronous rectifier in accordance with claim 1, further comprising:themeans contained in the control device for the determination of aninduction voltage through the first and second secondary windingscomprises a first resistor connected with the first connection of thecontrol device and which is connected to the fifth connection of thecontrol device by a a diode and a second resistor connected in parallelwith a second capacitor which are also connected with the secondconnection of the control device, whereby the voltage through the secondcapacitor is the value that is to be determined, the means of blockingthe first and second MOSFETs, whose gate electrodes are connected withthe connection point between the diode and the second resistor, a drainelectrode of a third MOSFET being connected with the fourth connectionof the control device, a drain electrode of a fourth MOSFET beingconnected with the third connection of the control device, a sourceelectrode of the third MOSFET being connected to the fifth connection ofthe control device by a third resistor and to the sixth connection ofthe control device, a source electrode of the fourth MOSFET beingconnected on a first side by a fourth resistor to the fifth connectionof the control device and on a second side being connected to theseventh connection of the control device; wherein when an inductionvoltage is present in the second secondary winding the gate electrodesof the third and fourth MOSFETs are continuously positive to the extentthat each of the third and fourth MOSFETs are conductive when thedrain-source voltage is correct, and the first and second MOSFETs workat the correct rhythm, and wherein when induction voltage of the thirdand fourth MOSFETs is below a select potential, the third and fourthMOSFETs are not conductive and the first and second MOSFETs work insubstrate-diode operation.
 3. The synchronous rectifier in accordancewith claim 1, further comprising:the means available in the controldevice for the determination of an induction voltage through the firstand second secondary windings comprises: a second diode and a fifthresistor connected in series with the third connection of the controldevice such that when the third connection of the control device has apositive polarity the second diode and the fifth resistor receivescurrent and charges a second capacitor connected to the fifth connectionof the control device; a third MOSFET having a source electrodeconnected to the sixth connection of the control device, and the fourthMOSFET having a source electrode connected to the seventh connection ofthe control device; a third resistor being connected between the fifthand the sixth connections of the control device, and a fourth resistorconnected between the fifth and the seventh connections of the controldevice; a zener diode having a cathode connected to a drain electrode ofthe third MOSFET and an anode connected to the fourth connection of thecontrol device; a galvanic connection between the gate electrodes of thethird and fourth MOSFETs and the positive terminal of the secondcapacitor; a second galvanic connection between the gate electrode ofthe fourth MOSFET and the first connection of the control device, andthird galvanic connection between the first and the fifth connections ofthe control device, whereby the second capacitor is always chargedpositively via the second diode and the fifth resistor if the voltagesthrough the first and second secondary windings are positive; whereinthe means for blocking the first and second MOSFETs further, in the caseof a breakdown of the induction voltages through the first and secondsecondary windings, discharges the gate electrodes of the third andfourth MOSFETs and the second capacitor via the substrate diodes of thethird and fourth MOSFETs, wherein the first and second MOSFETstransition into diode operation.